The present invention relates to a hydraulic brake system with slip control, in particular for automotive vehicles, comprising a master cylinder pressurizable by a hydraulic power booster, in which system valve means are inserted between the master cylinder and the wheel brakes connected to the master cylinder which serve to remove pressure fluid from the wheel brakes, wherein the pressure fluid taken from the wheel brakes can be replenished out of the pressure chamber of the hydraulic power booster, and wherein a stroke limitation of the brake pedal is effected during slip control.
A hydraulic brake system incorporating the preceding features is known from German printed and published patent application No. 3040562. This known brake system comprises a hydraulic power booster which is furnished with the pressure of an auxiliary energy source. Through a brake valve actuatable by a pedal, a hydraulic pressure can be built up in the pressure chamber of the hydraulic power booster which is proportional to the actuating force applied on the brake pedal. In the presence of a minimum pressure of response in the pressure chamber of the hydraulic power booster, which is essentially defined by the friction caused by the seals, the booster piston will start to move in the actuating direction and thus displace at the same time one or several master cylinder pistons in the actuating direction. Said master cylinder pistons serve to confine working chambers of a master cylinder which are in hydraulic communication with an unpressurized supply reservoir in the brake's release position. When the master cylinder pistons have performed a predeterminable travel, the communication to the unpressurized supply reservoir will be interrupted by suitable valve means. Therefore, continued movement of the master cylinder pistons in the actuating direction results in pressurization of the master cylinder chambers. Said master cylinder chambers are in hydraulic communication with wheel brakes of an automotive vehicle.
Inserted into the connections between the working chambers of the tandem master cylinder and the wheel brakes connected thereto are first valve means which are normally in the opened position and which are switchable by a slip control electronics to assume a closed position in which the wheel brakes are isolated from the master cylinder. Assigned to the wheel brakes are further valve means which are normally closed and which, when driven appropriately by a slip control electronics, will deliver pressure fluid out of the wheel brakes to an unpressurized supply reservoir. As the volume of pressure fluid enclosed in the working chambers of the tandem master cylinder is limited, it is necessary that pressure medium removed from the wheel brakes will be replenished.
For this purpose, the prior art brake system is furnished with a change-over valve controllable by the slip control electronics, which valve changes over on commencement of a control action and connects the pressure chamber of the hydraulic power booster to the pedal-side end faces of the master cylinder pistons. Said master cylinder pistons are sleeve-sealed and contain openings between their end faces so that the pressure medium supplied out of the pressure chamber of the hydraulic power booster will propagate via the sealing sleeves of the master cylinder pistons into the working chambers of the tandem master cylinder and into the wheel brakes, respectively.
In this prior art brake system, it must be provided that during slip control that a minimum reserve remains enclosed in the working chambers of the tandem master cylinder. To this end, the braking pressure generator disposes of a positioning sleeve which receives the booster piston of the hydraulic power booster and which is pressurized in the brake's release direction on actuating of the change-over valve. Said positioning sleeve will move thereupon opposite to the actuating direction, and after having made a specific stroke, will get into abutment on an annular collar of the booster piston. On account of the ratios of surfaces predefined by construction, no further depression of the brake pedal is possible in this operating condition. Besides, it may happen that the brake pedal is reset slightly via the booster piston. As a whole, the prior art brake system is of complicated design and necessitates relatively larger overall dimensions.
It is, therefore, an object of the present invention to improve upon the prior art braking pressure generator with structurally simple means such that a reduction of the necessary overall dimensions will result by utilization of the pre-charging effect known per se.